The present invention relates to electrographic imaging apparatus wherein a latent image is formed on a dielectric member and this image is toned to print out a permanent image on a recording sheet such as paper. Putting aside factors such as variation in toner composition, the amount of toner picked up by the imaging member depends on the charge distribution on the surface of the dielectric member, and this distribution may vary, for a given constant image pattern, due to environmental factors such as humidity and the aging or adjustment of various operating parameters of the machine. Thus, for example, in a photocopier, the charge may vary as the copy lamps or the photoconductive imaging member age and their characteristics change, or as the corona charging assembly and power supplies drift.
To compensate for variations in drum charge, it is common to provide a manual contrast control on such machines. Such a control may operate by shifting a corona charging potential, changing a transfer or bias potential used to transfer toner to the drum, or by some similar adjustment to compensate for the low or high density, or low or high contrast, which the charge distribution would normally produce.
In a like manner, it is also known to provide automated contrast or density control circuits which sense the charge on the drum and make similar adjustments to the lamp intensity, developing bias control or other such control parameter. Such sensors may sense the charge on a non-image position of the drum, or may identify a minimum value of the drum potential as the drum rotates past a sensor. It is also known to use an optical densitometer to sense the density of the toner which has adhered to the charged drum, and to make compensatory adjustments based on the sensed optical density, to some machine parameter.
In general, the setting up of an autocalibration system is believed to entail inspection of the amount of toner applied at a given setting for one or more particular fixed test patterns or test images, and special initialization procedures are used to evaluate and adjust system print darkness. Thus, these systems do not adjust print density on a-page-by-page basis for normal print images. A more general autocontrast control is therefore desirable.